Oxygen treatment of chromium alloys



Jam 1969 D. J. HANSEN OXYGEN TREATMENT OF CHROMIUM ALLOYS Sheet FiledFeb. 14. 1966 N fiumuxmmozk umawmwmm o. no 8 3 0 6 0 mood 60 0 R 0 o m x5:: 83 M \v El o .o v 0 so I- r|l 1|||\\ w A q 002 W 9N D 9. u. w d 002V i: -d J C P. |||||1I| lllll llll ll\lllll 3 k 0 u 00: 2 C 5 u. Q0 0o a\J OO we 2: B

ATTORNEY 7, 1969. D. J. HANSEN OXYGEN TREATMENT OF CHRQMIUM ALLOYS SheetFiled Feb. 14. 1966 $510856 umammumm R 0 0 0 00 0 0 0 80 0 60 0 m 88 m.o 0 m 09 90 4 A0 0 3M 0 0w 002 @T 00 v i 3 o w m N... 002 c 002 I m 0 0C 0 D \l v00: 2 C 0 0 00 d a o 002 u 002 Q 6$on 00 U. wkww DONALD J.HANSEN ATTORNEY Jan. 7, 1969 D. J. HANSEN OXYGEN TREATMENT OF CHROMIUMALLOYS Sheet 4 of 5 Filed Feb. 14. 1966 $513025: 3 mod umDmmumm 5 0 @006OOON N N Y WA R v m N -w A A my Jan. 7, 1969 n. J. HANSEN 3,420,657

OXYGEN TREATMENT OF CHROMIUM ALLOYS Filed Feb. 14, 1966 Sheet 5 of 5 Ya? o v o o 6. ole o v |0 0. u x

u 'o. o 1 u I o 2. g m a 2 o a Q Q 6 ii 2 I 3 i pl a. I -n g o l- C5. U5 I2 :3 5 m E 8 \1 \l \1 g Q o o 0 8 g g g a a x s BHI'IJNHI-HWBJ.

INVENTOR. wi- DONALD J. HANSEN ATTORNEY United States Patent 3,420,657OXYGEN TREATMENT OF CHROMIUM ALLOYS Donald J. Hansen, Lewiston, N.Y.,assignor to Union Carbide Corporation, a corporation of New York FiledFeb. 14, 1966, Ser. No. 527,214 U.S. Cl. 75-60 2 Claims Int. Cl. C22c3/00 ABSTRACT OF THE DISCLOSURE A process for rernOVing carbon fromiron-chromiumcarbon alloys without substantial loss of chromium whichcomprises: providing a substantially slag-free molten mass ofiron-chromium-carbon alloy, adjusting the starting composition of thealloy to provide between 75% and 15% chromium and a silicon content ofless than about 3% and treating the molten alloy 'by blowing oxygen onthe surface of said alloy, the pressure conditions at all times duringoxygen treatment, for a particular metal temperature, being in thevolume of the graph of the drawing which extends to the left of th planecorresponding to the chromium content of the alloy and in front of theplane corresponding to the desired final carbon content of the alloy.

The present invention relates to the oxygen treatment of chromiumallows. More particularly, the present invention relates to the oxygentreatment of chromiumiron-carbon alloys to provide such alloys with apredetermined chromium-carbon content.

The oxygen treatment of iron-chromiurn-carbon alloys at pressures belowatmospheric to remove carbon from the alloys has been known for sometime. However, the presently known techniques suffer from disadvantageswhich include the substantial undesirable loss of chromium, particularlywhen the effort is to achieve very low carbon levels. Moreover, usingpresently known oxygen techniques, control of the chromium-carbon ratioin the final alloy, and the production of high chromium, low carbonalloys has been found to be unpredictable at best.

It is therefore an object of the present invention to provide a processfor the oxygen treatment of chromiumiron-carbon alloys whereby thechromium-carbon ratio in the final alloy can be closely controlled.

It is another object of the present invention to provide an oxygentreatment for chromium-iron-carbon alloys whereby high chromium, lowcarbon alloys can be provided.

It is a further object of the present invention to provide anoxygen-treatment process for chromium-iron-carbon alloys wherebyundesirable chromium losses are avoided.

Other objects will be apparent from the following description and claimstaken in conjunction with the drawing which illustrates in FIGURES 1through temperatures and pressure conditions which are suitable for thepractice of the present invention.

A process in accordance with the present invention for adjusting thechromium-carbon content of iron-chromiumcarbon alloys to provide atleast a predetermined minimum chromium-to-carbon ratio in the finalalloy comprises providing a substantially slag-free molten mass ofironchromium-carbon alloy at a temperature of at least about 50 C. aboveits melting point; adjusting the composition of the alloy to the extentnecessary to provide a chromium content in the range of about to 75% anda silicon content of not more than 3 treating the molten alloy byblowing oxygen on the surface of said alloy and reducing the ambientpressure below atmospheric pressure to cause reaction of the oxygen andalloy, and

the evolution of reaction gases from the surface of the alloy thetemperature and pressure conditions being in accordance with theschedules more fully described hereinafter.

In the present invention, the alloys which can be effectively treatedrange from about 15 to Cr, up to 10% C balance iron and incidentalamounts of other elements such as Mn, S, Si, P, Ni, Cb, Ta, and Mo.

It is very important in the present invention that the silicon content,if any, of the alloy to be treated be less than about 3% by weight. Ifnecessary, the silicon content can be adjusted by the addition ofsuitable amounts of chromium, iron, or low-silicon iron-chromium alloywhen the alloy is in the molten state and prior to oxygen treatment.

The broad reason for maintaining the silicon content below the aforesaidvalue is to substantially avoid the formation of slag on the surface ofthe alloy during oxygen treatment. By operating essentially without slagon the surface of the alloy during oxygen treatment several significantadvantages are obtained. For eaxmple, recovery of chromium is improvedbecause any chromium that is inadvertently oxidized will not bedissolved in a slag with resultant loss in activity. In a slaglessprocess any chromium oxide formed can be readily redissolved withliberation of CO by suitable pressure reduction. A further increase inchromium recovery can be realized since no fine metal droplets will belost as suspended particles in a slag. Also for obtaining very lowcarbon alloys, the absence of a slag allows the surface of the moltenmetal to be completely exposed to the reduced pressure which promotesescape of CO gas with resultant lowering of the carbon content. On theother hand, with a slag present, CO gas can be trapped between themetalslag interface resulting in a higher CO partial pressure and lesseffective decarburization. Another important consideration favoring aslagless operation is the effect on maximum temperature of operation.The maximum temperature of an oxygen blowing operation is usuallylimited by the vessel refractories. When a slag cover is present,eutectics form between slag and the refractory lining lowering themaximum temperature of operation. The complete failure of arefractory-lined vacuum blowing vessel has been demonstrated when a slagcover was used under conditions that were completely satisfactory for aslagless operation. A still further advantage is higher efliciency ofoxygen since with a slag some of the oxygen will be deflected before itcomes in contact with the metal to be processed. Moreover the formationof a silicate slag requires addition of lime or some other material as aflux to make the slag fluid.

Most importantly, however, the avoidance of a silica slag permits theuse of particular processing schedules which enables the economical andaccurate compositional adjustment of iron-chromium-carbon alloy.

The figures of the drawing represent relationships between chromium andcarbon content for various temperature and pressure conditions duringoxygen treatment which have been found to be effective only when thesilicon content of the alloy being treated is below 3%.

With higher silicon contents, the illustrated relationships are notmeaningful due to the silica slag cover which develops and processingbecomes unpredictable.

With reference to the drawing, FIGURE 1 shows an orthogonal, threedimensional graph in which the surfaces indicated as 1 and 1 correspondto the chromium contents desired in the final product and are referredto herein as chromium planes. The two chromium planes illustrated inFIGURE 1 represent 70% Cr and 15% Cr which substantially covers therange of alloys suitably treated in accordance with the presentinvention. The

x, y, and z ordinates represent, respectively, ambient pressure duringoxygen treatment, the temperature of the molten alloy being treated andthe carbon content of the alloy. FIGURE 1 also shows at 3 and 3 theintersection of temperature planes with the chromium planes for varioustemperatures from 1600 C. to 2000 C.

FIGURE 2 shows the 70% chromium plane of FIG- URE 1 and the intersectiontherewith of carbon planes corresponding to carbon contents of 0.01%,0.05% and 0.1% and exemplary temperature planes. The remaining FIGURES3-5 show the chromium planes for 50%, 30% and chromium and therelationships therewith of carbon content, temperature, and pressure.Chromium planes for other Cr values can be readily obtained byinterpolation or extrapolation. In the practice of the presentinvention, a molten chromium-iron-carbon alloy is heated to initiallyprovide a temperature about 50 C. or more above its melting point. Themetal is transferred to a suitable vessel, for example a magnesia linedreactor which is placed within a vacuum chamber and fitted with H avacuum cover and oxygen is blown onto the surface of the molten metalfor example using to 70 s.c.f.m. per sq. ft. of molten metal surface.During the oxygen treatment, the pressure is reduced to belowatmospheric and controlled, together with the temperature of the moltenmetal surface as hereinafter described.

The ambient pressure over the metal bath can be adjusted by controllingthe rate at which gases are exhausted from the system, usingconventional vacuum equipment, preferably steam ejectors. Thetemperature of the metal surface is adjusted by altering the rate atwhich oxygen is reacted with the surface of the metal knowing theinitial temperature and the heat transfer characteristics of thereactor. The temperature can be suitably measured by an immersion typethermocouple or by a suitably adapted optical pyrometer.

By way of example, in the practice of the present invention, startingfor example with an iron-chromiumcarbon alloy containing 70% Cr and 1%C, to obtain a final alloy for 70% Cr and not more than 0.05 carbon,suitable pressure conditions employed during oxygen treatment lie in thevolume of the graph of the drawing which is to the left of the 70%chromium plane at a particular operating temperature and in front of thecarbon plane corresponding to 0.05 carbon.

Operating conditions to the right of the chromium plane will causeoxidation of chromium in pereference to carbon before the desired carbonlevel is reached.

More specifically, and with reference to FIGURE 2,

and the dotted arrows shown therein, oxygen treatment of a 70% Cr alloyat a metal temperature of 1800 C., and a pressure of 0.022 atmospheresprovides the selective oxidation and removal of carbon to 0.05% withoutsignificant oxidation of chromium i.e., not more than 5% of the chromiumis oxidized. Operation at a lower pressure, e.g. 0.01 atmospheres canprovide an even lower carbon content, as indicated at 5, before chromiumis significantly oxidized, or permit the use of a lower temperature, asindicated at 7, to obtain a carbon content of 0.05 without chromiumoxidation. Similarly, operation at a higher temperature will permit theuse of a higher pressure. In any event, throughout the practice of thepresent invention, for the operating temperature employed, the pressureshould be to the left of the intersection of the chromium plane and infront of the carbon plane corresponding to the carbon content of thealloy. This means that initial pressure can be higher than the finalpressure, but that under such circumstances, as the carbon content isdecreased, the pressure must be decreased in accordance with therelationship shown in the drawing.

The beginning of chromium oxidation, in preference to carbon oxidationcan be detected by a sharp decrease in the evolution of CO and CO fromthe surface of the molten alloy, regardless of the oxygen flow rate. Forexample, with oxygen flow rate held constant, when chromium begins tooxidize in preference to carbon, the decreased amount of reaction gaseswill cause a sudden noticeable decrease in pressure, and this phenomenamay be used in conjunction with the diagrams to control the compositionof the desired alloy.

In accordance with the preferred embodiments of the present invention,the conditions of temperature and pressure are controlled so that theycoincide with the intersection of the desired final carbon and chromiumplanes, and oxygen treatment is terminated when a sudden pressure dropis observed since further oxygen treatment would lead to additionaloxidation of chromium.

The FIGURES 3 through 5 as previously noted illustrate chromium planesfor 50%, 30% and 15 chromium respectively. Also shown in FIGURES 35 arecarbon plane intersections for 0.1%, 0.05% carbon and 0.01% carbon. Thegraphs of these figures, and FIGURE 2, can be used following theaforedescribed procedure to define oxygen treatment operating conditionsfor other alloys merely by interpolating or extrapolating when thechromium contents or intended operating temperatures do not correspondto the particular values illustrated.

In another embodiment of the present invention, the chromium/carbonratio of an iron-chromium-carbon alloy can be adjusted by the oxidationof chromium.

By way of example, the starting alloy can contain 16% chromium and 0.05%carbon, and the desired alloy is one containing 15% chromium and 0.05%maximum carbon. Molten alloy is provided at an initial temperature ofabout 1600 C., the pressure above the molten metal surface is reduced tobelow atmospheric, and oxygen is blown onto the surface of the alloy. Asa result of the oxygen treatment, the temperature of the surface of themetal rises. Since it is desired to reduce the chromium content of thealloy, the temperature and ambient pressure are controlled in accordancewith the present invention so as to be to the right of a 16% chromiumplane and substantially coincident with the intersection of the 15%chromium plane and the 0.05% carbon plane. It has been found that undersuch operating conditions, chromium is oxidized and removed from thealloy in preference to carbon. That is to say, the chromium will bepreferentially oxidized until the chromium content of the alloy is about15%, then chromium oxidation will abruptly cease and carbon will beoxidized. The commencement of carbon oxidation will be evidenced by theevolution of CO and CO from the surface of the molten alloy.

The following examples will further illustrate the present invention.

EXAMPLE 1 Two thousand six hundred twenty pounds of molteniron-chromium-carbon alloy at 1710 C. contained in a magnesia-linedreactor were positioned in a vacuum shell. The analysis of the alloy was66.8% chromium, 4.4% carbon, less than 0.5% silicon, balance iron. Thepressure in the vessel was reduced to 0.44 atmosphere by means of steamejectors, and 600 cubic feet of oxygen were introduced to the surface(3.40 sq. ft.) of the molten alloy through a water-cooled lance at arate of c.f.m. causing the temperature of the alloy to increase. Thepressure was reduced during the oxygen blow from 0.44 atmosphere to 0.21atmosphere, after which the vacuum vessel was backfilled with air andthe product sampled. The product analysis was 67.6% chromium, 2.7%carbon, less than 0.5 silicon, balance iron. Not more than 1.3% of theoriginal chromium was lost through oxidation.

EXAMPLE 2 Molten iron-carbon-chromium alloy weighing 4270 pounds at atemperature of 1600 C. and contained in a magnesia-lined reactor wasprovided in a vacuum shell. The analysis of the alloy was 0.38% carbon,19.4% chromium, less than 0.1% silicon, balance iron. The pressure inthe vessel was reduced to 0.275 atmosphere and 1250 cubic feet of oxygenwere introduced to the surface at 250 s.c.f.m. During the 5-minuteoxygen blowing period, the pressure was reduced to 0.01 atmosphere.After the oxygen was turned off, the alloy was held for an additional 5minutes at this pressure. The shell was backfilled with air, and theproduct analyzed 0.06% carbon, 18.4% chromium, balance iron.

EXAMPLE 3 Into a magnesia-lined reactor was poured 2100 pounds ofslag-free high-carbon ferrochrome analyzing 8.9% carbon, 63.5% chromium,less than 1% silicon, balance iron. The shell was closed and evacuatedto 500 mm. Hg pressure using steam ejectors. Approximately 400 cubicfeet of oxygen were introduced to the surface of the metal through awater-cooled lance according to the following schedule which wasselected first to keep the pressure of the shell to the left of thechromium plane in FIGURE 2, second to reduce the amount of gases beingevolved at lower pressures to minimize splashing, and third to providean oxygen rate to bring the temperature of the alloy to about 1800 C.

Shell pressure At this point in the oxygen blow, the pressure of thesystem dropped suddenly from 40 to 36 mm., indicating the beginning ofchromium oxidation. According to the graphs of the drawing 1800 C. and0.05 atmosphere pressure would begin at a carbon level of 0.2%. Since alower carbon was desired, an additional 500 cubic feet of oxygen at 80c.f.m. were introduced to the surface of the alloy, resulting inoxidation of chromium with formation of chromium oxide. The oxygen wasthen turned off and the pressure was reduced to 5 mm. which, accordingto the graphs of the drawing was below that necessary to obtain a 0.05%carbon alloy at 1800 C. The chromium oxide reacted with the carbon inthe alloy causing evolution of CO. When gas evolution had essentiallyceased (13 minutes), the shell was backfilled with air and the resultingproduct analyzed 0.04% carbon, 68.4% chromium. The chromium oxidationwas calculated to be 5%.

What is claimed is:

1. A process for removing carbon from iron-chromiumcarbon alloys withoutsubstantial loss of chromium which comprises:

(1) providing a substantially slag-free molten mass ofiron-chromium-carbon alloy at a temperature of at least about C. aboveits melting point (2) adjusting the starting composition of the alloy tothe extent necessary to provide between about and 15% chromium and asilicon content of less than about 3% (3) treating the molten alloy byblowing oxygen on the surface of said alloy and reducing the ambientpressure below atmospheric pressure to cause reaction of the oxygen andcarbon in the alloy and the evolution of reaction gases from the surfaceof said alloy, the pressure conditions at all times during oxygentreatment, for a particular metal temperature, being in the volume ofthe graph of the drawing which extends to the left of the planecorresponding to the chromium content of the alloy and in front of theplane corresponding to the desired final carbon content of the alloy.

2. A process in accordance with claim 1 wherein the oxygen blowing isterminated when the amount of gas evolved from the surface of the alloybeing treated suddenly decreases independently of the amount of oxygenbeing used in the oxygen blowing.

FOREIGN PATENTS 1,343,235 10/1963 France.

RICHARD O. DEAN, Primary Examiner.

U.S. Cl. X.R.

